Piezoelectric actuators are commonly used to control mechanical displacement of moving components in precision instruments. For example, a piezoelectric actuator may be used in an atomic force microscope (AFM) to control displacement of a cantilever probe as it measures a sample surface. Similarly, a piezoelectric actuator may be used in a hard disk drive (HDD) to control placement of a read/write head as it scans across a disk platter.
A piezoelectric actuator generally comprises a piezoelectric material e.g., a crystal, ceramic, etc.) that provides mechanical displacement in response to an applied electric field. In general, the mechanical displacement is proportional to an amount of electric charge applied to the piezoelectric material. However, due to the difficulty of directly controlling the amount of electric charge applied to the piezoelectric material, the mechanical displacement is typically controlled using a variable voltage source rather than a variable charge source.
FIG. 1 is a circuit diagram illustrating a conventional voltage-based approach to controlling a piezoelectric actuator 100. In FIG. 1, a piezoelectric actuator 100 comprises a high voltage amplifier 105 and a piezoelectric element 110. High voltage amplifier 105 provides an alternating current (AC) mode voltage to piezoelectric element 110, which causes it to expand and contract in a controlled manner. The AC mode voltage typically varies within a range of about 50-150 V, although it is not necessarily restricted to this range.
In general, the applied voltage can be related to the applied electrical charge by the equation q=Cv, wherein “q” represents the applied electrical charge, “v” represents the applied voltage, and “C” represents an intrinsic capacitance of the piezoelectric material over a specified range of frequencies. Unfortunately, however, the piezoelectric material does not behave like an ideal capacitor. Consequently, voltage-based control methods may lead to inaccurate control of the piezoelectric actuator.
Two ways that the piezoelectric material differs from an ideal capacitor include its characteristics of creep and hysteresis. Creep refers to a change in displacement over time without any change in the control voltage. For example, the piezoelectric material may initially expand by 95% of a desired increment after receiving the control voltage, and then it may slowly expand, or creep, by another 5% after receiving the control voltage. This creep can be explained by slower charge migration compared with an ideal capacitor. Hysteresis, on the other hand, refers to state dependent behavior of the piezoelectric material. For example, the piezoelectric material's response to changes in the control voltage may vary depending on how much it is currently contracted or expanded.
In light of these and other characteristics of piezoelectric materials, there is a need for improved techniques to control piezoelectric actuators with greater accuracy, stability, and speed.